When I first started a number of riveting troubles were caused by the parts not being clamped or secured very well. If the part is wobbling around and you are trying to support it, use the bucking bar and rivet gun... well, it doesn't work too well. If the parts are clamped and secured, you can see how everything lines up and you can get the bucking bar square to the rivet, then things go a lot easier.

I use various thicknesses of rubber/neoprene sheet. I cut a small piece; ~1/2" square is usually plenty. I drill a hole near the edge using the same drill I'd use for the rivet hole. Then cut from edge into the hole using scissors or wire cutters. The hole ends up smaller than the rivet, but will slip over the rivet tail easily. I use a thickness that's roughly equal to tail length that's exposed when the stack is tight. A short burst with the gun will clinch the stack, then the rubber is removed & setting is finished.

A rivet should be stet with as few as possible blows from the gun. If not it will work harden to the point it will fail and pop the head off. That was a fellows problem on here a few months back. He could not figure out why it was happening thought it was bad rivets.
Just a thought when driving them. Boyd in Chiloquin Or

On my VS I used my hand squeezer with the handles on the aft side of the front spar. I put the rivets in with the manufactured head on the front side. I think I had to bend the nose rib upward a bit in order to get the yoke onto it but not too much. One nice thing about the hand squeezer is you can go slowly (compared to pneumatic squeezer or rivet gun) to better control setting the shop head on the aft side of the spar, and by pulling the handles of the squeezer aft you can keep enough pressure on the manufactured head to keep the nose rib flange flush to the spar, no o-rings or other gadgets required...

When I had my fin inspected by the MDRA (Canada) I had a bit of a gap between the rib and the spar. They snagged it and I sent a picture to Vans. they answered that the purpose of those rivets was simply to hold the rib in place and that the loads were transmitted to the spars mostly via the skin. So, if you have the rib not lying down tight to the spar it is not a strength issue, it is only a "fussiness" issue.

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